1. Field of the Invention
The present invention relates to a hydrometallurgical process for a nickel oxide ore, and in more detail, the present invention relates to a hydrometallurgical process for a nickel oxide ore, which is capable of reducing use amount of hydrogen sulfide gas in a sulfurization step and use amount of an alkali to be used in exhaust gas treatment, and decreasing operation cost, by enhancement of utilization efficiency of hydrogen sulfide gas, while maintaining nickel recovery rate to a high yield of equal to or higher than 95%, and preferably equal to or higher than 98%, in a hydrometallurgical process for a nickel oxide ore including:
a step (1) for obtaining an aqueous solution of crude nickel sulfate by High Pressure Acid Leach of a nickel oxide ore, a step (2) for obtaining zinc sulfide and a zinc free final solution formed by introduction of the above aqueous solution of crude nickel sulfate into the inside of a sulfurization reactor (A), then the addition of hydrogen sulfide gas, a step (3) for obtaining a mixed sulfide of nickel/cobalt and a waste solution by introduction of the above zinc free final solution into the inside of a sulfurization reactor (B), then the addition of hydrogen sulfide gas, and a step (4) for scrubbing treatment of hydrogen sulfide gas in exhaust gas generating in the above steps (2) and (3).
2. Description of the Prior Art
A High Pressure Acid Leach using sulfuric acid has been noticed in recent years, as the hydrometallurgical process for a nickel oxide ore. This method is composed of wet process steps throughout, without dry process treatment steps such as drying and roasting steps and the like, thus providing advantages not only in view of energy and cost saving but also in being capable of obtaining a mixed sulfide of nickel/cobalt having an enhanced nickel content of up to about 50% by weight.
As the High Pressure Acid Leach for obtaining the above mixed sulfide of nickel/cobalt, for example, there has been used a method including: a step (1) for obtaining an aqueous solution of crude nickel sulfate containing zinc as an impurity element, in addition to nickel and cobalt, by High Pressure Acid Leach of a nickel oxide ore, a step (2) for obtaining zinc sulfide and a zinc free final solution formed, by introduction of the above aqueous solution of crude nickel sulfate into the inside of a sulfurization reactor (A), then the addition of hydrogen sulfide gas, sulfurization of zinc contained in the aqueous solution of crude nickel sulfate, and then solid-liquid separation, a step (3) for obtaining a mixed sulfide of nickel/cobalt and a waste solution, by introduction of the above zinc free final solution into the inside of a sulfurization reactor (B), then the addition of hydrogen sulfide gas, sulfurization of nickel and cobalt contained in the zinc free final solution, and subsequently introduction of slurry formed into an evaporation apparatus for evaporation of hydrogen sulfide gas, and then solid-liquid separation, and a step (4) for scrubbing treatment of hydrogen sulfide gas in exhaust gas generating in the above steps (2) and (3).
FIG. 1 shows an example of a process chart of a hydrometallurgical process for a nickel oxide ore according to a High Pressure Acid Leach.
In FIG. 1, a nickel oxide ore 5 is firstly subjected to High Pressure Acid Leach using sulfuric acid to form leached slurry, in the step (1). Next, the leached slurry is subjected to solid-liquid separation, and after multi-stage washings, separated to a leachate containing nickel and cobalt, and a leaching residue 7. The above leachate is subjected to neutralization to form the neutralized precipitate slurry containing a trivalent iron hydroxide, and an aqueous solution 6 of crude nickel sulfate. After that, the aqueous solution 6 of crude nickel sulfate is subjected to the sulfurization step composed of the step (2) and the step (3), and separated to a zinc sulfide 9 and a zinc free final solution 8, and a mixed sulfide 10 of nickel/cobalt and a waste solution 11, respectively. A sulfurization reactor to be used in this sulfurization step, is usually composed of a closed-type reactor equipped with a supply port of a reaction starting solution, an outlet of slurry after the reaction, a charge hole of hydrogen sulfide gas, and an exhaust gas hole.
It should be noted that exhaust gas 12 containing hydrogen sulfide gas generating from the step (2) and the step (3) is introduced into a scrubber of a step (4), and it is subjected to contact with the alkaline aqueous solution for absorption of hydrogen sulfide gas. The resulting waste solution from the scrubber obtained here is treated separately. Still more, a waste solution 11 is circulated to be used as a washing solution in solid-liquid separation in the step (1).
Here, the above step (1) is composed of a leaching step for obtaining leached slurry, by the addition of sulfuric acid into slurry of a nickel oxide ore and leaching at a high temperature of equal to or high than 200° C. under high pressure using an autoclave, a solid-liquid separation step for separation to the leaching residue in leached slurry and a leachate containing nickel and cobalt, and a neutralization step for forming the neutralized precipitate slurry containing impurity elements such as iron, and a starting solution for a sulfurization reaction, by adjustment of pH of the leachate containing impurity elements, in addition to nickel and cobalt.
In addition, in the above steps (2) and (3), a sulfurization reaction is carried out by the addition of hydrogen sulfide gas into the aqueous solution of crude nickel sulfate containing zinc as an impurity element, in addition to nickel and cobalt, to form a metal sulfide. Therefore, enhancement of efficiency of the sulfurization reaction is important.
As for the enhancement of efficiency of this sulfurization reaction, the following sulfurization methods have been disclosed. For example, a method for controlling the sulfurization reaction of metals by using hydrogen sulfide gas as a sulfurizing agent and adjusting concentration of hydrogen sulfide in a vapor phase, and correctly controlling ORP or pH in a solution (for example, refer to Patent Literature 1), a method for the addition of a sulfide seed crystal to promote the sulfurization reaction, as well as to suppress the adhesion of a generating sulfide onto the inner surface of the reactor (for example, refer to Patent Literature 2), and a method for separation of zinc preferentially, by adjustment of pH and ORP of the aqueous solution of nickel sulfate containing cobalt and zinc (for example, refer to Patent Literature 3) and the like. These conventional technologies are effective technologies to solve each of the problems, even in the above High Pressure Acid Leach.
Incidentally, as the operation method of the above step (3), for example, operation is carried out under control of operation conditions such as nickel concentration, introduction flow amount, temperature, pH of a reaction starting solution to be introduced into the sulfurization reactor, at predetermined values, by blowing the hydrogen sulfide gas having a hydrogen sulfide gas concentration of equal to or higher than 95% by volume into the vapor phase inside the sulfurization reactor and controlling the inner pressure thereof at predetermined value, and also, if necessary, by the addition of the sulfide seed crystal. This way enabled to secure a nickel recovery rate of equal to or higher than 95%. However, in order to enhance the nickel recovery rate stably at a still higher level, it is considered to carry out the reaction in a state of more increased temperature and pressure inside the sulfurization reactor. This case raises problems of use amount of hydrogen sulfide gas, along with treatment cost of exhaust gas from a reaction system, or cost of a reaction apparatus, therefore enhancement of utilization efficiency of hydrogen sulfide gas to be added to the sulfurization step is required, to solve these problems. However, there is no description, in the above conventional technology, on enhancement of utilization efficiency of hydrogen sulfide gas.
Still more, in a production facility of hydrogen sulfide gas to be used industrially in a plant of a hydrometallurgical process for such as a practical operation plant of the above High Pressure Acid Leach, it is advantageous, in view of production efficiency thereof, to produce and use gas having a hydrogen sulfide gas concentration of below 100% by volume. Therefore, in hydrogen sulfide gas to be added inside the sulfurization reactor, hydrogen of a raw material in the production step of hydrogen sulfide gas, or an inert component such as nitrogen commingling in the production step of hydrogen sulfide gas, is contained in an amount of about 2 to 3% by volume. That is, hydrogen or nitrogen is included as an inert component not involved in the sulfurization reaction.
Therefore, in continued implementation of the operation in the sulfurization step such as the above steps (2), (3), the above inert component is accumulated inside the sulfurization reactor, causing decrease in sulfurization reaction efficiency. Therefore, such an operation is carried out that gas inside the sulfurization reactor is periodically discharged outside the system. In this case, because not only the inert component but also residual hydrogen sulfide gas are discharged at the same time, as exhaust gas, loss of hydrogen sulfide gas generates. In addition, exhaust gas from the inside of this sulfurization reactor essentially requires scrubbing treatment such as absorption of hydrogen sulfide gas, for example, by subjecting to contact with an alkaline aqueous solution, therefore increase in use amount of hydrogen sulfide gas increases use amount of the alkali. As countermeasures thereof, it is considered to decrease vapor phase pressure or concentration of hydrogen sulfide inside the sulfurization reactor, however, this countermeasures, as described above, raises a problem of making it difficult to secure a nickel recovery rate of equal to or higher than 95%, which is a minimal level necessary as efficiency of total operation, and preferably equal to or higher than 98%.
Under these circumstances, in a practical operation plant of the conventional High Pressure Acid Leach, a nickel recovery rate of equal to or higher than 95% in a mixed sulfide of nickel/cobalt, has been secured by the excess addition of use amount of hydrogen sulfide gas, in a degree of about 1.3 to 1.4 time hydrogen sulfide amount required theoretically in view of the sulfurization reaction. Therefore, such a method has been required that is capable of reducing the use amount of hydrogen sulfide gas in the sulfurization step, and the use amount of the alkali to be used in exhaust gas treatment, and decreasing operation cost, while maintaining the nickel recovery rate of equal to or higher than 95%.    [Patent Literature 1] JP-A-2003-313617 (page 1 and page 2)    [Patent Literature 2] JP-A-2005-350766 (page 1 and page 2)    [Patent Literature 3] JP-A-2002-121624 (page 1 and page 2)